Brian Spurlock-USA TODAY Sports
Ankle flexion is most important in defensive edge-rushers, and this trait is on display consistently during their position drills.
When a defensive lineman can show ability to "bend" in bag drills and change-of-direction assignments, scouts will look to see where the bend comes from. Coming off the edge, having good ankle flexion—torque from feet through ankles—is a major plus with scouts.
Ankle flexion is necessary in an edge-rusher's ability to "flatten" in their speed-to-power conversion rush. The main thing to look for is how flat a player's foot is on the ground when planting and changing directions with a bend.
It isn't that scouts want players to be flat-footed, they just want to see where the potential power of an inside move is capable of stemming from. This requires surface area and torque.
If the player shows the ability to preserve the power that comes from driving off the maximum surface area their feet take up on the ground through their ankles, that is a major pass-rush indicator.
When a player comes off the edge with a speed rush, the most important piece is what that player converts the speed rush to.
To convert the speed rush "inside to power"—toward the quarterback—the player who can get low and flatten through this move without losing the power harnessed through their plant foot is ideal.